Method of hot-dip metallising metal strips



Nov. 27,1962 A. BUSCH 3,066,041

METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25. 1960 4Sheets-Sheet 1 INVENTOR.

AND/PEAS 5115c ATTORNEYS Nov. 27, 1962 A. BUSCH 3,066,041

v METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25, 1960 4Sheets-Sheet 2 FIG}.

HVTOR. AND/F545 BUS C A/ A TORNEYS Nov. 27, 1962 A. BUSCH 3,066,041

METHOD OF HOT-DIP METALLISING METAL STRIPS Filed Jul 25, 1960 v 4Sheets-Sheet :5

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ATTORNEYS 4 Sheets-Sheet 4 mmvron 1/1/0254: 3050/ A. BUSCH Nov. 27, 1962METHOD OF HOT-DIP METALLISING METAL STRIPS Filed July 25, 1960 H A: w 1

ATTORNEYS he States In the known procedure for hot-dip metallisingstrips of steel or metal, the steel strip passes first through a fluxand then through a metal melt, for instance of tin, whereafter thesurplus metal is retained by means of squeezing rolls which run in agrease bath placed immediately above the metal vat. This known procedurehas considerable disadvantages, more particularly so far as hot-diptinning is concerned. Out-put is very low, since a strip can passthrough the tin bath and through the squeezing rolls, which run in palmoil, at a speed of only about 2.4 m./min. The speed cannot be increased;if it is, the thickness of the tin coating becomes excessive. Thethickness of the tin cannot be controlled accurately and reliably. Thetin layer is relatively thick and varies over a range of about -40 g./m.The thickness of the tin is uneven and a variation of i3 g./m. is likelyin a strip. The tin layer is also nonuniform-i.e., porous-so thathot-diptinned strips or plates do not resist corrosion well. Anotherdisadvantage is that a number of pairs of squeezing rolls running inpalm oil must be provided. The squeezing rolls must be made of steel,and the roll periphery becomes covered with a tin film, the thickness ofwhich governs the thickness of the tin layer applied to the strip. Also,the steel strips has to be given a complex degreasing treatment afterbeing tinned.

The alternative methods which have been proposed for hot-dip metallisinghave not led to any satisfactory result. It is an object of thisinvention to provide a considerable increase in the speed of the stripbeing treated, and consequently to improve output considerably. Anotherobject of the invention is, notwithstanding a high strip speed, toreduce the thickness of the metal coating or to provide selectivecontrol of such thickness. A final object of the invention is, despite ahigh strip speed, to provide satisfactory, uniform and non-porous metalcoverings. A high strip speed means considerable difficulties incompletely removing surface impurities, more particularly metal oxides.According to an earlier proposal by the applicants which has notpreviously been published, the strip, which is with advantage preheated,is first given conventional hot-dip metallising treat1nenti.e., it ispassed through a flux bath into a metal melt--whereafter the strip whichissues from the melt passes through squeezing rolls which preferablyhave a metal-repellant surface, such rolls running exclusively in aprotective gas atmosphere and being supplied, before the strip entersthe roll gap, with a reducing agent, preferably a gaseous flux. Thismethod enables the strip to move very fast, for the squeezing rolls donot run in a liquid. However, after some time has elapsed the squeezingrolls become soiled by the metal oxides or reduction residues carriedalong by the strip, with the result that the strip surface is impaired.One reason for this is that the strip moves so fast through the fluxbath that the same cannot completely deoxidise the strip, and reductionresidues are carried along through the melt.

To obviate these disadvantages, in the method according to the inventiona metal intermediate layer, such as tin, as first applied to the strip.Then the strip is heated to a temperature above the melting point of theintermediate layer, preferably in an inert or reducing atmosphere.

3,6dtifil4l Patented Nov. 27, 1962 ice Then the strip is introduced intoa metal melt, such as tin, and a flux, preferably gaseous, is sosupplied to the strip before the entry of the strip into the metal meltthat the dissolved impurities, more particularly metal oxides, floataway on the concave wetting meniscus formed, by the metal melt on thestrip. The impurities are removed continuously, from the strip as itenters the melt. The strip issuing from the melt is conveyed inaprotective gas atmosphere to the squeezing rolls which rolls run exclusively in such atmosphere. The advantage of this novel method is thatthe provision of a metal intermediate layer leads to the stripcomprising relatively few impurities, more particularly surface metaloxides, and even they can be readily removed according to the novelmethod when the strip dips into the metal melt at high speed. Anotherfactor enabling the strip to run at a high speed is that the surplus tinis removed from the tin issuing from the bath by means of squeezingrolls which run exclusively in a protective gas atmosphere. If required,the reliability of the novel method can be further improved by supplyingthe squeezing rolls with a non-liquid and preferably gaseous flux. Thesqueezing rolls used have a metal-repellent and preferably resilientsurface layer. The squeezing roll surface can consist of chromium-platedsteel or carbon or porcelain or quartz glass. Strip speeds of about 40m./min. are possible with such rolls, but if silicone rubber is used forthe roll surface, strip speeds of up to m./min. are possible. The rollpressures are much less than the high roll pressures associated with theknown squeezing rolls running in palm oil-only about 20 kg. for a rollwidth of 600 mm.

In the hot-dip leading of steel strips it is known for the strip firstto be tinned by galvanising, whereafter the tin layer is fused to thestrip in a continuous furnace in a reducing atmosphere, and onlythereafter does the strip pass into a lead melt. The pretinning andfusion are necessary in this hot-dip leading method to provide anintermediate layer which consists of an alloy of iron and tin and whichmust be provided if the layer of the lead is to adhere satisfactorily.In this known procedure the surplus lead is squeezed off the strip bymeans of squeezing roll pairs running in palm oil, with the result thatstrip speed is very low. Also, the reducing atmosphere in the continuousfurnace does not sufiice to remove the metal oxides from the strip,consequently a flux bath must, as a rule, be provided in conventionalmanner on the entry side of the lead melt. Even if sufficient pretinningis provided to enable a flux bath to be omitted, continuous operation isimpossible. At high strip speeds, the amount of impurities produced isso great that their removal is not technically feasible.

The invention will hereinafter be described in greater detail withreference to the drawings which illustrate embodiments and in which:

FIG. 1 is a diagrammatic view of the complete apparatus for metallisingmetal strips;

PEG. 2 is a detailed view of the squeezing roller arrangement used inthe apparatus shown in FIG. 1;

FIG. 3 is a view to an enlarged scale showing the entry of the stripinto the metal melt of FIG. 1;

\FIG. 4 shows a variant of the apparatus in FIG. 3; and

FIG. 5 shows an alternative arrangement for carrying the novel methodinto effect.

Referring to FIG. 1, a metal or steel strip is unwound from a stripreel 1. The following are arranged in the path followed by the stripfrom the reel 1 to a taking-on reel 16: a Welder 2; a retarder 3; apickling device 4; a brushing and washing device 5; a galvanising plant6 for applying a metal intermediate layer; a brushing and washing device7; a continuous furnace 8 in which the strip is preheated by heaters 9and and the metal intermediate layer which has been applied by thegalvanising plant 6 is fused to the strip; a metal melt 10 through whichthe strip passes; squeezing rolls 11 for removing surplus metal; acooling duct 12 for cooling the strip; a tank 13 for aftertreatment(passivation) of the strip; a drying duct 14 and a drawing device 15.The remainder of the description refers by way of example to the tinningof a steel strip.

The strip is provided in the galvanising plant 6 with a thin tin layerof only about 0.5-2 g./m. thickness (on both sides). This intermediatelayer is fused on to the strip in the continuous furnace in an inert orreducing protective gas atmosphere and (FIG. 3) the steel strip 17coated with this liquid tin intermediate layer 18 is introduced into thetin melt It). Inductive heaters 9 or other means can be used to heat thestrip in the furnace S. Conveniently, the inert protective gas issupplied to the bottom of the continuous furnace at 19. The impuritiesstill present on the strip surface, more particularly metal oxides, areindicated by the references 2%). A concave meniscus 21 consisting ofliquid metal (It?) forms where the strip dips into the melt 10.Consequently the impurities can float away on this meniscus, in thedirection indicated by arrows, on to the surface of the melt 10. Toensure that the impurities, more particularly metal oxides, arecompletely removed from the strip surface, supply ducts 22 are providedon either side of the strip for a preferably gaseous flux, such ashydrogen chloride, which issues from fine apertures 23-i.e., very nearthe meniscuses 21. Such flux, rising in ducts 24 on either side of thestrip, is indicated by the reference 25. As is apparent from thedrawings, the exit duct 24 widens into a hood 27 which dips into themelt 10 at 26, the flux supply ducts 22 being disposed on the hood,preferably in the transition region between the duct 24 and the hood 27.Advantageously, the walls of the exit duct 24 are at a very reduceddistance a from the strip 17, and the hood 27 is similarly at the veryreduced distance a from the surface of the metal melt. The distance a isfrom about 3 to 5 mm. Intimate engagement of the flux with the stripsurface and the melt is therefore ensured.

The impurities 2% which float off on the meniscus at 21 are removedcontinuously from the surface of the melt It). To this end (FIG. 3),there are provided on either side of the strip inside the hood 27 arotating steel roller 28 which dips into the melt, a scraper 29 which ismade of asbestos or the like and which co-operates with the roller 28and a collecting box 30 which also dips into the melt. The rollers 23,which rotate slowly in the direction indicated by arrows, remove theimpurities continuously from the melt surface and convey the impuritiesto the boxes 30.

The hood 27 and duct 24 can be moved vertically relatively to thefurnace 8 so that the hood can be lifted in order to be cleaned or for acheck to be made of the meniscus 21.

In the novel method there is no layer of flux, such as is usually found,on the metal melt l0, consequently it is easy to check the meniscus 21to see whether the impurities are being removed satisfactorily. Ifrequired, the impurities can be removed from the melt 19 in a differentway, for instance, as shown in FIG. 4, where collecting boxes 32 whichdip into the melt and which each comprise an overflow 31 for theimpurities 2d are provided one each on either side of the strip inside ahood 27a; disposed in each box 32, with the interposition of a filter 33of glass wool or the like, is a pump 35 which is driven by a motor 34-and which conveys to the melt ltl, through a return pipe 35, the metalwhich has spilt into the box through the overflow pipe 31. Using thisoverflow principle, impurities which float off the strip 21 can beremoved reliably at high strip speeds.

If required, the method according to the invention can be carried intoeffect using an arrangement of the kind shown in FIG. 5 wherein acontinuous furnace Sa in which the strip 17 moves upwards is provided tofuse the metal intermediate layer 18 to the strip 17. Flux supply ducts38 are disposed on either side of the exit channel 37 of the furnace 8a,and collecting tanks 39 which receive the impurities 20 floating off themeniscus at 21 are disposed below the ducts Immediately above the duct37 are supply pipes 49 through which the metal melt 41 is supplied andwhich extend to near the strip at 42. The melt 41 can be supplied bycapillary action, the melt being supplied to the supply pipes 46 from asupply tank 43. Disposed above the pipes 4-1 is a casing which receivessqueezing rolls 44 and to which an inert protective gas is suppliedthrough pipes 46. Preferably, a flux, for instance, hydrogen chloride,is supplied in small amounts through pipes 47 to the strip before thesame enters the roll gap.

A similar squeezing roll arrangement is provided in the apparatus shownin FIG. 1 and is illustrated in FIG. 2. Referring to FIG. 2, thesqueezing rolls 11 run in an inert gas atmosphere in a casing 43. Theinert gas is supplied through pipes 49 to a duct 5% which dipspcrpendicularly into the melt ltl. Pipes 51 through which a gaseous fluxcan be supplied open into the duct 50 immediately below the roll gap.

The quantities of flux supplied at 22 and 38 at the place where thestrip enters the melt are relatively small; similarly, the gas pressurein the pipes 22 or 38 is relatively low and is only about 5-20 mm.water.

The novel method hereinbefore described has the advantage that,nothwithstanding high strip speeds, uniform and non-porous metalcoverings can be produced. Fairly thin tin layers of up to 15 g./m. canbe provided in hot dip tinning using the novel method. The thickness ofthe metal layer can be varied as required by varying the surfacequalities of the tin-repelling squeezing rolls. Metal coatings of veryreduced thickness can be produced by means of polished squeezing rolls.

A great advantage of the premetallising step, which can be formed notonly by galvanising but, if required by diffusion or pasting or thelike, is that the immersion times in the metal melt can be very short,with the result of a further advantagei.e., a reduction of the alloylayer. Since the immersion times are so very short, the vesselscontaining the melt It can be very small.

Another advantage of the method according to the inventioni.e. of thepremetallising stepis that the melt can consist of metals not alloyablewith the basic metal of the strip, since an appropriate metal can beused for the intermediate layer in the premetallising step. In hot dipleading, therefore, the steel strip can be provided with an intermediatelayer of tin, while when a steel strip is hot-dip metallised withaluminium, a zinc intermediate layer can be provided. The invention istherefore not limited to the hot-dip tinning which has been mentionedsolely by way of example.

I claim:

1. A method of hot-dip metallizing metal strips which comprises:applying a metal intermediate layer to the st ip; heating said strip toa temperature above the melting point of said metal intermediate layerin a nonoxidizing atmosphere; applying a flux to the heated strip andthen passing the strip through a metal melt, the molten metal forming aconcave wetting meniscus where the strip enters the metal so that thedissolved impurities on the strip are floated away therefrom; and movingthe strip exiting from the bath through a protective gas atmosphere tosqueezing rolls which operate in the protective gas atmosphere.

2. A method according to claim 1 wherein the flux applied to the stripis in a gaseous form.

3. A method according to claim 1 including the step of applying anon-liquid flux to the strip after it exits from the molten metal andbefore it enters between the squeeze rolls.

4. A method according to claim 1 wherein the squeeze rolls have a metalrepellent surface layer.

5. A method according to claim 4 wherein the surface layers of thesqueeze rolls are resilient.

6. A method according to claim 1 wherein the strip is of steel, theintermediate layer consists essentially of tin and the flux is hydrogenchloride.

7. Apparatus for the hot'dip metalization of metal strips, comprising: afurnace through which the strip is forwarded, said furnace having anentrance end and an exit end; flux supply ducts adjacent to the exit endof the furnace for applying a flux to said strip; molten metal applyingmeans closely adjacent to said flux supply ducts for applying moltenmetal to said strip; means for removing impurities from the zone wherethe molten metal is applied to the strip; a casing closely adjacent saidapplying means so that the strip passes directly from said applyingmeans to said casing, said casing having rotatable squeeze rolls thereinbetween which the strip passes; and means for maintaining a protectivegas atmosphere in said casing.

8. Apparatus for the hot-dip metalization of metal strips, comprising: afurnace through which the strip is forwarded; a metal bath located belowthe furnace; an exit conduit extending from the lower end of the furnacetoward the bath; a hood connected to the lower end of the exit conduitand extending sidewardly therefrom, said hood overlying the uppersurface of the metal in said bath, said hood having edge portions spacedlaterally from said exit conduit which edge portions extend downwardlyinto said bath; means for supplying a flux to the interior of saidconduit and said hood, said last-named means being located Where saidhood is connected to said exit conduit; and means for supplying aprotective gas to the lower end of the furnace.

9. Apparatus according to claim 8, in which the internal walls of theexit conduit and the hood are located close to, but are spaced from, thestrip and the upper surface of the metal in said bath, respectively.

10. Apparatus according to claim 9, including a pair of rotatablerollers located underneath and spaced from the hood, said rollers beinglocated on opposite sides of the strip, said rollers being partiallysubmerged in the bath so that the upper portion of the periphery thereofis above the upper surface of the bath whereby impurities on the uppersurface of the bath are carried out of the bath on the peripheries ofsaid rollers; a scraper for removing impurities from the peripheries ofthe rollers; and a collecting box associated with each roller forreceiving the impurities removed therefrom by the scraper, thecollecting box also extending partway into the molten metal.

11. An apparatus according to claim 9, including a pair of collectingboxes located underneath and spaced from the hood, the collecting boxesbeing located on opposite sides of the strip, the collecting boxes beingat least partially submerged in the bath, said boxes each having anoverflow on the surface of molten metal through which impurities on thesurface of the bath may enter said collecting boxes; a filter withineach of the boxes for separating the impurities from the metal which mayflow into the collecting boxes therewith; and a pump for transferringthe metal separated in the collecting box back to the bath.

12. Apparatus for the hot-dip metalization of metal strips, comprising:a furnace through which the strip is forwarded, the strip being movedupwardly through the furnace; an exit conduit extending upwardly fromthe upper end of the furnace; flux supply ducts for supplying flux tosaid exit conduit; collecting boxes for receiving impurities removedfrom the strip, said collecting boxes being disposed below the fluxsupply conduits; metal sup ply ducts located above and closely adjacentthe upper end of said exit conduit for supplying molten metal to saidstrip as it exits from said exit conduit; a casing having rotatablesqueeze rolls therein between which the strip passes, said casing beingdisposed above and adjacent to said metal supply ducts; and means forsupplying a protective gas to said casing.

References Cited in the file of this patent UNITED STATES PATENTS2,216,519 Quarnstrom Oct. 1, 1940 2,224,578 Wean et al. Dec. 10, 19402,405,592 Manger et al. Aug. 13, 1946 2,459,161 Harris et al. Jan. 18,1949 2,656,285 Burns et al. Oct. 20, 1953 2,761,793 Brennan Sept. 4,1956 2,914,423 Knapp Nov. 24, 1959

1. A METHOD OF HOT-DIP METALLIZING METAL STRIPS WHICH COMPRISES:APPLYING A METAL INTERMEDIATE LAYER TO THE STRIP; HEATING SAID STRIP TOA TEMPERATURE ABOVE THE MELTING POINT OF SAID METAL INTERMEDIATE LAYERIN A NONOXIDIZING ATMOSPHERE; APPLYING A FLUX TO THE HEATED STRIP ANDTHEN PASSING THE STRIP THROUGH A METAL MELT, THE MOLTEN METAL FORMING ACONCAVE WETTING MENISCUS WHERE THE STRIP ENTERS THE METAL SO THAT THEDISSSOLVED IMPURITIES ON THE STRIP ARE FLOATED AWAY THEREFROM; ANDMOVING THE STRIP EXITING FROM THE BATH THROUGH A PROTECTIVE GASATMOSPHERE TO SQUEEZING ROLLS WHICH OPERATE IN THE PROTECTIVE GASATMOSPHERE.